The development of low loss, fused silica lightguide fiber over the last few years has led to the investigation of high temperature (e.g., approximately 2000.degree. C.) heat sources, for the drawing of high strength fiber from a lightguide preform. Of the possible heat sources, the oxy-hydrogen torch, the CO.sub.2 laser and a few induction and resistance furnaces have been employed for drawing the high silica fibers. The torch method, while inexpensive, cannot maintain a uniform diameter over long lengths of fiber. The CO.sub.2 laser provides the cleanest drawing atmosphere, but requires special optical designs to radially distribute the energy for drawing and is limited in power, while resistance furnaces require an inert, protective, atmosphere to prevent oxidation of the heating element.
An induction furnace is basically comprised of a hollow, centrally located, tubular susceptor surrounded by insulating material. An induction coil is mounted about the insulating material to provide an alternating electromagnetic field when energized. The field couples to the susceptor, elevating the temperature thereof, forming a hot zone therein. A glass lightguide preform is then introduced into the hot zone to reflow a portion thereof from which a lightguide fiber is drawn.
High temperature induction furnaces provide high thermal inertia, high stability, and radially symmetric heating. Most induction furnaces, however, use graphite or refractory metallic susceptors which require the flowing of protective atmospheres during operation to remove contaminants migrating from the inside surface of the susceptor. As a result such furnaces have limited susceptor life and have some degree of contamination in the furnace atmosphere.
One induction furnace designed to overcome the foregoing problems is described in an article entitled "A Zirconia Induction Furnace For Drawing Precision Silica Waveguides" by R. B. Runk in the Optical Fiber Transmission II technical digest (Tu B5-1), Feb. 22-24, 1977 which is incorporated by reference herein. That furnace uses a cylindrical susceptor tube made of yttria stabilized zirconium dioxide. The susceptor has a long life expectancy and has minimal furnace atmosphere contamination in an oxygen-bearing atmosphere. Such a furnace has been shown to be most effective in drawing lightguide fiber from a preform. However, it has been found that after extended use microscopic particles of zirconium dioxide migrate from the susceptor onto the preform and/or the fiber being drawn from the lightguide preform. The zirconium dioxide particles substantially weaken the drawn fiber resulting in unacceptable product.
One technique used to substantially eliminate the migration of the zirconium dioxide particles from the susceptor tube is described in copending U.S. patent application Ser. No. 383,066 to U. C. Paek and C. M. Schroeder Case 14-4 entitled "Modified Zirconia Induction Furnace" which was filed on even date herewith. That application is assigned to the instant assignee and is incorporated by reference herein. The Paek-Schroeder application describes the deposition and consolidation on the inside surface of the susceptor of a porous "soot" of the same material (e.g., silica) as that of the lightguide preform to be heated therein. The porous soot may be consolidated as the temperature of the furnace is raised to the drawing temperature of approximately 2000.degree. C. Such a technique has been most successful under normal running conditions which tend to cause cracks in the susceptor on the order of about 0.015 inch in width or less. However, when the cracks are greater than that thickness the migration of particulate zirconium dioxide resumes. This appears to be due to such cracks being too large to be filled with the consolidated soot, which permits microscopic particles from the insulating zirconium dioxide grain, which surround the susceptor, to be drawn therethrough and deposit on the preform and/or the drawn fiber.
Accordingly, there is a need for a technique for substantially eliminating contaminating particulates from migrating through the relatively large cracks in the furnace susceptor.